I like the mockup, but I’m not sure if the green, yellow and red colors are useful, or maybe I don’t understand what they indicate.
Another option (to avoid the cross origin issue) is to allow the discourse server to do the talking with the hub instead of the browser, like we currently do when checking for available usernames.
A security concern is: are you who you say you are? If a Discourse site pushes a change for niceguy@example.com to set his name to “Stinky Face”, was it really him who made that push? The simplest solution would be for the hub to send an email to Stinky Face niceguy asking him to click a link to verify the change.